Technical Field
The present disclosure relates to a display device, in which touch sensors and pixels are embedded in a display panel, and a gate driver circuit of the display device.
Description of the Related Art
User interfaces (UIs) are configured to allow users to communicate with various electronic devices and to easily and comfortably control the electronic devices as they desire. Examples of UIs include a keypad, a keyboard, a mouse, an on-screen display (OSD), and a remote controller having an infrared communication function or a radio frequency (RF) communication function. User interface technology has continuously expanded to increase user's sensibility and handling convenience. UIs have been recently developed to include touch UIs, voice recognition UIs, 3D UIs, and the like.
A touch UI senses a touch input using a touch screen implemented on a display panel and transmits the touch input to an electronic device. The touch UI has been adopted in portable information devices, such as smart phones, and use of the touch UI has been expanded to include uses in computer monitors and home appliances.
A technology for implementing a touch screen has been recently applied to various display devices using a technology (hereinafter referred to as “in-cell touch sensor technology”) for embedding touch sensors in a pixel array of a display panel. The touch sensors may be implemented as capacitive touch sensors sensing a touch input based on changes in a capacitance before and after the touch input.
In in-cell touch sensor technology, touch sensors may be installed in a display panel without an increase in a thickness of the display panel. As shown in FIG. 1, in the in-cell touch sensor technology, a common electrode for supplying a common voltage Vcom to pixels of a liquid crystal display may be divided to form touch sensor electrodes (including, for example, touch sensor electrodes C1 to C4 shown in FIG. 1). The touch sensor electrodes (including, e.g., C1 to C4) are connected to sensor lines SL. Because touch sensors Cs are embedded in a pixel array of a display panel, the touch sensors Cs are coupled with pixels through parasitic capacitances. In order to reduce signal interference (e.g., crosstalk) attributable to coupling between the pixels and the touch sensors Cs in the in-cell touch sensor technology, one frame period is time-divided into a display period and a touch sensing period. The in-cell touch sensor technology supplies a reference voltage (i.e., the common voltage Vcom) of the pixel to the touch sensor electrodes (including, e.g., C1 to C4) during the display period and drives the touch sensors Cs and senses a touch input during the touch sensing period.
A display device includes a data driver supplying a data voltage to data lines of a display panel, a gate driver (also referred to as a gate driver circuit or a scan driver) supplying a gate pulse (also referred to as a scan pulse) to gate lines of the display panel, and a touch sensing unit (also referred to as a touch sensing circuit or a touch driver circuit) driving touch sensors.
The gate driver sequentially shifts the gate pulse applied to the gate lines using a shift register. The gate pulse is synchronized with the data voltage (i.e., a pixel voltage) of an input image and sequentially selects respective pixels to be charged with the data voltage. The shift register includes cascade-connected stages. The stages of the shift register receive a start signal or a carry signal received from a previous stage as the start signal, and generate an output when a clock is input.
A screen of the display device may be divided into two or more blocks, and a touch sensing period may be allocated between a driving time of one block and a driving time of another block. For example, during a first display period, pixels of a first block may be driven, and data of the first block may be updated to current frame data. During a touch sensing period following the first display period, a touch input may be sensed. During a second display period following the touch sensing period, pixels of a second block may be driven, and data of the second block may be updated to current frame data. However, such a method may deteriorate output characteristic of the gate pulse supplied to the gate lines, and as a result, lead to a reduction in image quality of the display device.
For example, in the second block driven immediately after the touch sensing period, a voltage of a Q node at a stage of a shift register outputting a first gate pulse may be discharged during the touch sensing period due to a leakage current. Because the Q node of the stage is connected to a gate of a pull-up transistor, a decrease in the voltage of the Q node may be generated and may make a bootstrapping operation of turning on the pull-up transistor incomplete. Hence, the gate pulse, of which a voltage rises due to the pull-up transistor, does not rise to a normal voltage level. As a result, a luminance of pixels arranged on a first line of the second block may be reduced due to a decrease in a voltage of the first gate pulse generated when the pixels of the second block start to be driven, and a reduction in the image quality, such as a line dim, may appear. In the shift register, in which an output of a previous stage as a carry signal is input to a start signal input terminal of a next stage, a reduction in output characteristic of the stage generating the first gate pulse after the touch sensing period leads to a decrease in voltages of all of gate pulses generated after the first gate pulse. Further, there is no gate pulse generated after the first gate pulse.
In the in-cell touch sensor technology, a touch sensing period and a display period following the touch sensing period are fixed in every frame period. Because of this, a position of a stage, of which a Q node is charged during the touch sensing period, is always the same at the shift register of the gate driver. In each stage of the shift register, a gate of a pull-up transistor is connected to a Q node. As a result, a pull-up transistor connected to a Q node in stages of the shift register, each of which temporarily stops to operating due to the touch sensing period, receives more DC gate bias stress than pull-up transistors of other stages. In a transistor of a metal oxide semiconductor field effect transistor (MOSFET) structure, the DC gate bias stress deteriorates the transistor, leading to a change in a threshold voltage or output characteristic of the transistor.
When one frame period is time-divided into a display period and a touch sensing period in a display device including in-cell touch sensors, a line dim phenomenon, in which lines are visible at a particular position of the screen, may be generated. In the display device including the in-cell touch sensors, a shift register outputting a gate pulse maintains a Q node in a charged state during the touch sensing period. Hence, the DC gate bias stress of pull-up transistors increases. The reliability of the shift register is reduced due to the DC gate bias stress, and lifespan of the shift register is shortened.